Adhesion promoters have long been used in liquid inks to improve adhesion and resistance properties to enable the liquid ink formulations to meet the requirements of the customer and the end-user. The main purposes of the adhesion promoter is to improve adhesion to prevent removal of ink from the substrate and to prevent potential contamination of the packaged product due to ink removal. Titanium based compounds are widely used in liquid inks to improve the adhesion of the color system to substrates such as flexible packaging. Adhesion promoters to date produce the desired properties after a specified dwell time, for example 24 hours or longer, and impart limited adhesion to some substrates, especially those with low surface tension and coated substrates, for example those coated with aluminum oxide and silica oxide.
Those titanium based compounds may be used without a synergist, as described for example in U.S. Pat. Nos. 4,094,853, 4,087,402, 4,659,848, WO04053003 and U.S. Pat. No. 7,619,021, or with a synergist, as disclosed in U.S. Pat. Nos. 4,725,501, 4,909,846 and 5,116,411.
Adhesion promoters currently employed in liquid inks for flexible packaging typically have the following characteristics and performance properties:                Impart adhesion of an ink or coating after a specified period of time, such as for example, 24 hours or longer.        Require the application of elevated temperature to reduce the time to impart adhesion or improved adhesion.        Provide limited adhesion to some untreated substrates or substrates with low surface tension, for example those with a surface tension of less than 38 dynes/cm.        Impart limited adhesion to certain coated plastic substrates such as, but not limited to those coated with aluminum oxide, silica oxide, PVdC, Acrylic, PVOH, Nitrocellulose.        Create odor issues, for example odor associated with the release of acetyl acetone (2,4-pentanedione) present in titanium acetyl acetone adhesion promoters; the release of propionic acid when using zirconium propionate adhesion promoter and the release of amylic alcohol.        Require additional resins such as but not limited to high acid value additives, (i.e. greater than 100 mg/KOH), to minimize and/or control thickening in the ink or coating medium.        Require the use of specific solvents, for example higher alcohols, for stability.        Impart discoloration, for example yellowing problems in white inks associated with titanium acetyl acetone adhesion promoters, high acid value resins used to stabilize organo-titanates.        Can reduce lamination performance if used at higher levels (i.e. the level of addition required to bring about desired adhesion may reduce lamination performance).        In examples of lamination failure, ink removal or transfer may be viewed as a mode of failure.        Usage levels needed to obtain desirable adhesion often leads to a compromise of other ink performance properties.        
WO 2005/049662 discloses an adhesion-modified acid functional polyamide resin including a non-yellowing titanate chelate having amyl acid phosphate ligands. Due to the potential for amyl acid phosphate to breakdown to amylic alcohol, the adhesion modifier in WO 2005/049662 can create an odor.
Traditional adhesion promoters are effective in crosslinking hydroxy-functional resins (for example nitrocellulose, polyvinyl butyral, cellulose acetate butyrate, polyamide, polyurethane, etc.) commonly employed in commercial liquid inks to impart improved adhesion to plastic substrates. WO 2004/053003 describes the general mechanism of adhesion as occurring via a condensation reaction between titanium alkoxy groups and polymer hydroxyls. WO 2004/053003 also explains that the mechanism of thickening in the ink is due to a tendency for a crosslinking reaction to occur in the coating formulation during storage or during mixing. Those skilled in the formulation of liquid inks have traditionally employed the use of high acid value materials, for example, maleic resin, dodecyl benzene sulphonic acid, ortho-phosphoric acid, dimerised rosin, etc., with an acid value in excess of 25 mg KOH/g and more preferably in excess of 100 mg KOH/g to reduce the undesirable thickening during storage. Thickening is determined by measuring the viscosity of the ink at the time of the final quality control check after manufacture and comparing this to the viscosity at a later time, for example after a period of 24 hours, several days, months or even years. Viscosity can be measured using any number of viscometers or viscosity flow cups or can be compared by manually stirring with an ink knife or spatula and noting the viscosity difference. In the present application, viscosity stability was assessed by measuring viscosity using a DIN 4 viscosity flow cup immediately after ink manufacture and then taking comparative viscosity readings after 24 hours at 25° C. A rise in viscosity after manufacture is considered disadvantageous (see Table 5).
The use of high acid value materials can present undesirable side effects such as a reduction in water/moisture barrier properties, discoloration and a reduction in printability performance. The novel adhesion promoters of the present invention exhibit improved stability and do not require the use of high acid value resins, thereby allowing simpler formulations with improved properties.